1. Field of the Invention
This invention relates generally to filtering devices, and more particularly to a rotary type of filtering device which is of the general type employed in the pulp and paper industry.
2. Description of the Prior Art
For years rotary filter devices have been used in various applications, notably in the pulp and paper industry. Such devices are cylindrical in shape and have been constructed of metallic substances, typically stainless steel, or other metal clad with stainless steel. An outer surface is composed of a filtering medium, typically a mesh composed of metal or plastic. Vacuum or other suction forces applied from within the filter cause the substance being filtered, for example, pulp saturated with water, chemicals, or various fluids, to adhere to the filter medium and draw the fluids out of the pulp. The pulp is then removed as the filter nears completion of a rotation cycle, typically by means of a scraping device selectively positioned with respect to the filter surface, or by application of air pressure. Rotation rate of the filter is set to match filter diameter, pulp concentration, and other variables such that the desired degree of filtration is achieved by the time the filter nears completion of one rotation cycle. The filter is positioned so that the lower part thereof dips into a container of the saturated pulp. Thus the filtering process is a continuous process, with the filtered pulp being removed from the filter just before the filter surface rotates back through the container, and as it rotates through the container suction forces cause the saturated pulp to adhere to the outer surface and repeat the cycle.
Prior art devices have generally consisted of an inner cylindrical surface having a plurality of support structures which extend radially outward from the cylinder surface, and extend longitudinally across the entire length of the inner surface. The support structures are grooved or notched to receive a spirally wound wire continuously wrapped around them. These winding wires form a partial surface to support the filtering medium. In prior art devices the support structures and winding wire have consisted of a metallic substance, typically stainless steel. The device is then covered with a pliable mesh filtering medium, which forms the outer surface of the filter.
Although many problems exist in connection with use of such a prior art device, these devices have been used without substantial change or improvement for more than twenty years. In use, the various exposed surfaces of the filter are in continuous contact with various corrosive substances, such as chlorides, which cause all the exposed metallic surfaces to corrode. These filtering devices are typically of enormous size, such as ten feet or more in diameter, and represent a substantial capital investment, typically a few hundred thousand dollars. Yet when used in a corrosive environment, repairs and modifications are frequently necessitated, which require shutting down the paper machine, or other process, for significant periods of time. Each time the machine is shut down it means a loss of many thousands of dollars, and the greater the length of time the machine is down the more the loss.
In typical use, within a few years it will be necessary to refinish the filter, because of corrosion. The grooves in the winding wire support structures will have corroded to the point that they have been virtually eaten away, and no longer furnish a firm support for the winding wires. The winding wires are then free to move and slide about, making it impossible to maintain uniformity of the filtering process. The rough surfaces will cause frequent breakage of the winding wires, and frequent rips and tears in the filtering medium, and each time these incidents occur it requires the process to be stopped until the necessary repairs can be made. It becomes necessary to remove the filtering medium, remove the winding wire, and grind down and regroove the winding wire support structures. The stainless steel winding wires also present a safety hazard, as they must be spirally wound onto the drum under tension, and if breakage occurs people in the area may receive injury.